1. Technical Field
The invention is concerned with methods for the manufacture of composite materials consisting of finely powdered filler in a polymer matrix, and new composite materials made by such methods. It is concerned particularly with methods for the manufacture of such composite materials having very flat surfaces suitable for the reception of electrical conductors, and with such flat surfaced products made by such methods.
2. Background Art
The electronics industry is an example of one which makes substantial use of flat surfaced substrates as supports for electronic circuits, such substrates consisting of thin flat pieces produced to exacting specifications as to starting material and physical and electrical properties. The history of the industry shows the use of progressively higher operating frequencies and currently for frequencies up to about 800 megahertz (MHz) copper coated circuit boards of glass fiber reinforced polymers, such as epoxies, cyanide esters, polytetrafluoroethylene (PTFE) and polyimides, have been and are still used. For frequencies above 800 MHz the current materials of choice are certain ceramics formed by sintering suitable powdered inorganic materials, such as silica; alumina; aluminum nitride; boron nitride; barium titanate; barium titanate complexes such as Ba(Mg1/3Ti2/3)O2, Ba(Zr,Sn)TiO4, and BaTiO3 doped with Sc2O3 and rare earth oxides; alkoxide-derived SrZrO3 and SrTiO3; and pyrochlore structured Ba(Zr,Nb) oxides. Substrates have also been employed consisting of metal and semiconductor powders embedded in a glass or polymer matrix, a particular preferred family of polymers being those based on PTFE.
For example, ceramic substrates that have been used for hybrid electronic circuit applications comprise square plates of 5 cm (2 in) sides, their production usually involving the preparation of a slurry of the finely powdered materials dispersed in a liquid vehicle, dewatering the slip to a stiff leathery mixture, making a xe2x80x9cgreenxe2x80x9d preform from the mixture, and then sintering the preform to become the final substrate plate. The substrates are required to have high values of uniformity of thickness, grain size, grain structure and density, with the purpose of obtaining substantially uniform dielectric, thermal and chemical properties, and high values of surface flatness and surface finish to permit the uniform application to the surfaces of fine lines of conductive or resistive metals or inks.
Such sintered products inherently contain a number of special and very characteristic types of flaws. A first consists of fine holes created by the entrainment of bubbles in the ceramic pre-casting slip of sizes in the range about 1-20 micrometers; these bubbles cannot be removed from the slip by known methods and cause residual porosity in the body. As an example, sintered alumina substrates may have as many as 800 residual bubble holes per sq/cm of surface (5,000 per sq/in). Another flaw is triple-point holes at the junctions of the ceramic particles when the substrate has been formed by roll-compacting of spray-dried powder; they are of similar size to the bubble holes and appear in similar numbers per sq/cm. Yet another consists of xe2x80x9cknit-linesxe2x80x9d, which are webs or networks of seam lines of lower density formed at the contact areas between butting particles during cold pressing. Two other common flaws are caused by inclusions of foreign matter into the material during processing, and the enlarged grains caused by agglomeration of the particles despite their initial fine grinding. The usual inclusions are fine particles due to abrasive wear of the grinding media in the mills. Both inclusions and agglomerates will sinter in a matrix at a different rate from the remainder of the matrix and can result in flaws of much greater magnitude than the original inclusion or agglomerate.
Costly mirror-finishing by diamond machining and lapping of these prior art ceramic surfaces is required to allow the accurate deposition of sputtered metal layers from which conductor lines are formed by etching. Mirror-finishes are required because the electrical currents at frequencies above 0.8 GHz move predominantly in the skin of a conductor line, while in the lower frequencies they occupy the entire cross-section. The thickness of the skin through which currents move at GHz frequencies becomes thinner as frequencies rise and are already as thin as 1 to 2 micrometers in copper at around 2 GHz. Any surface roughness of the conductors will therefore contribute to conductive losses. For example, at a frequency of 4 GHz, the conductive loss at of the interface between conductor and substrate is 1.65 times higher with an RMS roughness of 40, compared to an RMS roughness of 5 (See P.42 of Materials and Processes for Microwave Hybrids, R. Brown, published 1989 by the International Society for Hybrid Microelectronics of Reston, Va.)
There is therefore continuing interest in methods for manufacturing composite materials for the production of electronic substrates with which the major surfaces are as flat as possible, while sintering and its attendant difficulties, and the considerable costs of diamond machining and lapping are avoided.
The principal object of the invention is therefore to provide new methods for manufacturing composite materials consisting of particles of finely powdered filler material bonded together in a matrix of polymer material, such new composite materials, and articles made from such composite materials, wherein such materials and articles can more readily be produced with surfaces of the flatness demanded by the electronics industry.
It is another object to provide such new methods with which the resultant composite materials and articles comprise at least 60 percent by volume of filler material, with the remainder consisting of polymer matrix, wherein again such materials and articles can readily be produced with surfaces of the flatness demanded by the electronics industry.
It is a further object to provide such new methods which are operable to produce composite materials and articles comprising at least 60 percent by volume of filler material, with the remainder consisting essentially of polymer matrix, wherein again the materials are produced in a form with which the surfaces intended for the reception of electric conductors and the like are inherently readily producible with surfaces of the flatness demanded by the electronics industry.
In accordance with the invention there are provided methods for the manufacture of composites of finely powdered fillers in a polymer matrix comprising the steps of:
forming a solution of the polymer in volatilizable solvent;
mixing filler material particles with sufficient solution to form a suspension having therein the balance in volume percent of the polymer required for the composite;
evaporating solvent from the suspension while subjecting it to high shear treatment so as to maintain high values of uniform distribution of filler particles in the solution, the evaporation being continued until a mixture is obtained consisting essentially of filler particles with the residual solution distributed substantially uniformly therein, the mixture being of consistency suitable for production of thin coherent layers;
producing thin coherent layers from the mixture; continuing evaporation of solvent from the thin coherent layers until it has substantially entirely been removed;
placing a stack of a plurality of the thin coherent layers in a mold in sufficient number to obtain a composite article of the desired thickness; and
subjecting the stack of thin coherent layers to a temperature sufficient to melt the polymer material and to a pressure sufficient to unite the layers and to maintain the melted polymer material substantially uniformly dispersed in the interstices between the filler material particles.
Also in accordance with the invention there are provided articles consisting of bodies of composite materials comprising finely powdered filler material particles substantially uniformly distributed in a polymer matrix;
wherein each body comprises a composite mixture of filler material particles and the balance polymer, the polymer being soluble in a volatizable solvent that has been volatilized from the mixture;
wherein each body comprises a stack of a plurality of united thin coherent layers in number sufficient to provide a body of the desired thickness; and
wherein the body has been formed from the stack of thin coherent layers by subjecting the stack to a temperature sufficient to melt the polymer and to a pressure sufficient to unite the layers and disperse the melted polymer into the interstices between the filler material particles to a high degree of uniformity.